A major goal in solid organ transplantation is the permanent engraftment of the donor organ without a graft rejection immune response generated by the recipient, while preserving the immunocompetence of the recipient against other foreign antigens. Typically, in order to prevent host rejection responses, nonspecific immunosuppressive agents such as cyclosporine, methotrexate, steroids and FK506 are used. These agents must be administered on a daily basis and if stopped, graft rejection usually results. However, a major problem in using nonspecific immunosuppressive agents is that they function by suppressing all aspects of the immune response, thereby greatly increasing a recipient's susceptibility to infections and other diseases, including cancer.
Furthermore, despite the use of immunosuppressive agents, graft rejection still remains a major source of morbidity and mortality in human organ transplantation. Most human transplants fail within 10 years without permanent graft acceptance. Only 50% of heart transplants survive 5 years and 20% of kidney transplants survive 10 years. (See Opelz et al., 1981, Lancet 1:1223; Gjertson, 1992, UCLA Tissue Typing Laboratory, p. 225; Powles, 1980, Lancet, p. 327; Ramsay, 1982, New Engl. J. Med., p. 392). It would therefore be a major advance if tolerance to the donor cells can be induced in the recipient.
The only known clinical condition in which complete systemic donor-specific transplantation tolerance occurs is when chimerism is created through bone marrow transplantation. (See Qin et al., 1989, J Exp Med. 169:779; Sykes et al., 1988, Immunol. Today 9:23; Sharabi et al., 1989, J. Exp. Med. 169:493). This has been achieved in neonatal and adult animal models as well as in humans by total lymphoid irradiation of a recipient followed by bone marrow transplantation with donor cells. The success rate of allogeneic bone marrow transplantation is, in large part, dependent on the ability to closely match the major histocompatibility complex (MHC) of the donor cells with that of the recipient cells to minimize the antigenic differences between the donor and the recipient, thereby reducing the frequency of host-versus-graft responses and graft-versus-host disease (GVHD). In fact, MHC matching is essential, only one or two antigen mismatch is acceptable because GVHD is very severe in cases of greater disparities. In addition, it also requires the appropriate conditioning of the recipient by lethal doses of total body irradiation (TBI).
The MHC is a gene complex that encodes a large array of individually unique glycoproteins expressed on the surface of both donor and host cells that are the major targets of transplantation rejection immune responses. In the human, the MHC is referred to as HLA. When HLA identity is achieved by matching a patient with a family member such as a sibling, the probability of a successful outcome is relatively high, although GVHD is still not completely eliminated. However, when allogeneic bone marrow transplantation is performed between two MHC-mismatched individuals of the same species, common complications involve failure of engraftment, poor immunocompetence and a high incidence of GVHD. Unfortunately, only about 20% of all potential candidates for bone marrow transplantation have a suitable family member match.
The field of bone marrow transplantation was developed originally to treat bone marrow-derived cancers. It is believed by those skilled in the art even today that lethal conditioning of a human recipient is required to achieve successful engraftment of donor bone marrow cells in the recipient. In fact, prior to the present invention, current conventional bone marrow transplantation has exclusively relied upon lethal conditioning approaches to achieve donor bone marrow engraftment. The requirement for lethal irradiation of the host which renders it totally immunocompetent poses a significant limitation to the potential clinical application of bone marrow transplantation to a variety of disease conditions, including solid organ or cellular transplantation, sickle cell anemia, thalassemia and aplastic anemia.
The risk inherent in tolerance-inducing conditioning approaches must be low when less toxic means of treating rejection are available or in cases of morbid, but relatively benign conditions. In addition to solid organ transplantation, hematologic disorders, including aplastic anemia, severe combined immunodeficiency (SCID) states, thalassemia, diabetes and other autoimmune disease states, sickle cell anemia, and some enzyme deficiency states, may all significantly benefit from a nonlethal preparative regimen which would allow partial engraftment of allogeneic or even xenogeneic bone marrow to create a mixed host/donor chimeric state with preservation of immunocompetence and resistance to GVHD. For example, it is known that only approximately 40% of normal erythrocytes are required to prevent an acute sickle cell crisis (Jandl et al., 1961, Blood 18(2):133; Cohen et al., 1984, Blood 76(7):1657), making sickle cell disease a prime candidate for an approach to achieve mixed multilineage chimerism. Although the morbidity and mortality associated with the conventional full cytoreduction currently utilized for allogeneic bone marrow transplantation cannot be justified for relatively benign disorders, the induction of multilineage chimerism by a less aggressive regimen certainly remains a viable option. Moreover, the use of bone marrow from an HIV-resistant species offers a potential therapeutic strategy for the treatment of acquired immunodeficiency syndrome (AIDS) if bone marrow from a closely related species will also engraft under similar nonlethal conditions, thereby producing new hematopoietic cells such as T cells which are resistant to infection by the AIDS virus.
A number of sublethal conditioning approaches in an attempt to achieve engraftment of allogeneic bone marrow stem cells with less aggressive cytoreduction have been reported in rodent models (Mayumi and Good, 1989, J Exp Med 169:213; Slavin et al., 1978, J Exp Med 147(3):700; McCarthy et al., 1985, Transplantation 40(1):12; Sharabi et al., 1990, J Exp Med 172(1):195; Monaco et al., 1966, Ann NY Acad Sci 129:190). However, reliable and stable donor cell engraftment as evidence of multilineage chimerism was not demonstrated, and long-term tolerance has remained a question in many of these models (Sharabi and Sachs, 1989, J. Exp,.Med. 169:493; Cobbold et al., 1992, Immunol. Rev. 129:165; Qin et al., 1990, Eur. J. Immunol. 20:2737). Moreover, reproducible engraftment has not been achieved, especially when multimajor and multiminor antigenic disparities existed.
Permanent tolerance to donor antigens has been documented in H-2 (MHC) identical or congenic strains with minimal therapy and/or transplantation of donor skin drafts or splenocytes alone (Qin et al., 1990, Eur J Immunol 20:2737). However, similar attempts to achieve engraftment and tolerance in MHC-mismatched combinations have not enjoyed the same success. In most models, only transient donor-specific tolerance has been achieved (Mayumi et al., 1987, Transplantation 44(2):286; Mayumi et al., 1986, Transplantation 42(4):417; Cobbold et al., 1990, Eur J Immunol 20:2747; Cobbold et al., 1990, Seminars in Immunology 2:377).
Early work by Wood and Monaco attempted to induce tolerance using bone marrow plus anti-lymphocyte serum (ALS) in partial MHC-matched donor-recipient combinations (Wood et al., 1971, Trans Proc 3(1):676; Wood and Monaco, 1977, Transplantation (Baltimore) 23:78). Even in this semi-allogeneic system, F.sub.1 splenocytes were required to facilitate the induction of tolerance, and thymectomy was required for stable long-term tolerance. The additional requirement for splenocytes and thymectomy made potential clinical applicability of such an approach unlikely. However, these studies identified two key factors required for the induction of tolerance: an antigenic source of tolerogen, which is not only involved in tolerance induction, but must also be present at least periodically for permanent antigen-specific tolerance, and a method to tolerize, or prevent activation of new T cells from the thymus, i.e. thymectomy, or intrathymic clonal deletion.
Attempts to induce tolerance to allogeneic bone marrow donor cells using combinations of depleting and non-depleting anti-CD4 and CD8 monoclonal antibodies (mAb) resulted in only transient tolerance to MHC-compatible combinations (Cobbold et al., 1992, Immunol Rev 129:165; Qin et al., 1990, Eur J Immunol 20:2737). 6 Gy of TBI was required to obtain stable engraftment and tolerance when MHC-disparate bone marrow was utilized (Cobbold et al., 1986, Transplantation 42:239). Sharabi and Sachs attributed the failure of anti-CD4/CD8 mAb therapy alone to the inability of mAb to deplete T cells from the thymus, since persistent cells coated with mAb could be identified in this location (Sharabi and Sachs, 1989, J Exp Med 169:493). However, subsequent attempts to induce tolerance by the addition of 7 Gy of selective thymic irradiation prior to donor bone marrow transplantation also failed. Engraftment was only achieved with the addition of 3 Gy of recipient TBI. Therefore, there remains a need for non-lethal methods of conditioning a recipient for allogeneic bone marrow transplantation that would result in stable mixed multilineage allogeneic chimerism and long-term donor-specific tolerance.